Custom Validation Script To Set Drop-down Items

I am trying to use a custom validation script (or a custom keystroke script - whatever is best) in a drop-down to set the items in another drop-down based on the display value (not the export value).  If I use the export value, it works without issue.  If I use the display value, it is always one step behind the selection.  I am using the following validation script in Dropdown1:

if(event.target.getItemAt(event.target.currentValueIndices, false)=="A")

{

this.getField("Dropdown2").setItems(["aa","aaa"])

}

if(event.target.getItemAt(event.target.currentValueIndices, false)=="B")

{

this.getField("Dropdown2").setItems(["bb","bbb"])

}

When I select "A" in dropdown one, it sets the items as if I selected "B" and vice versa.